disclaimer: just an old grumpy customer, not an industry expert.
There were pointless debates around ITU-T and 3GPP as to whether the LTE is 3.5G or 3.9G or 4G next to the pink fact elephant that _it_ is going to be _the_ next cell standard anyway. That hot debate delayed voice call discussions to post-launch matter that eventually coagulated as the VoLTE.
The Voice-over-LTE was a total kludge together that were(ARE) carrier specific implementations. In Japan at least, it seemed to have had push from KDDI, esp. with hindsight that NTT docomo and Fujitsu, both formerly influential in 3G, both seemed on fire majority of that timeframe, while Nokia being a supernova beyond fire. It would be very natural if Samsung would have made vital contributions, but I don't have much informational pressure from that direction. VoLTE is phone company mannerisms, remnants of retro-futuristic media features of 3G, weird spaghetti codes from carrier labs, and international call exchange system, all homogenized in a blender; the artefacts were co-developed pair of server and client implementations that are standardized in the way there aren't many implementations but not something carefully spec-worked before construction for interoperability. That all happened ~a decade ago. Some point between Android 4 to 10.
I suppose it was all retconned into 3GPP standards after action, but the whole stack is still like a lobotomized Android call app and private fork of Asterisk embedded into the cellular core monstrosity. I guess embedded modem people(like Quectel) had finally got to port functions into modem chip firmwares so they can make calls, after someone done it for non-smartphones(KaiOS, Smarterphone...), I think around 2019 +/- 1 year.
What I'm trying to say is, VoLTE is complicated. It's something like SIM-authenticated SIP/RTP over IP under IP, not even regular 3G data session let alone SIP-VoIP on Layer 3 UDP/IP.
Osmocom project Wiki summarizes it better than I[1]: "Voice over LTE is an adaptation by 3GPP to use IMS over an LTE cellular network. The LTE EPS (Evolved Packet System) provides the functionality of the underlying IP-CAN. ... IMS is much more than normal SIP/RTP. And in addition to that, there is a tight integration between the LTE system and the IMS on top of it."
And it's supposed to be the core feature of a phone, "not".toupper() a party trick... There are better things to do in life for most people.
1: https://osmocom.org/projects/foss-ims-client/wiki/VoLTE